For the croquettes: To remove the salt, place the cod in a dish of 8 cups cold water and leave in the refrigerator for 24 hours, changing the water a couple of times.
Bring the milk and garlic to a simmer in a saucepot. Add the cod fish and cook for about 10 minutes. Drain and set aside.
Place the potatoes in a pot and cover with salt water. Bring a boil and cook until fork tender, 10 to 15 minutes, and then drain.
Heat the olive oil in a saute pan and saute the shallots until the edges are crispy and golden, about 3 minutes. Transfer to a big bowl with the cod and potatoes. Mash together, and then stir in the mayonnaise, mustard, parsley, hot sauce to taste, if using, and some salt and pepper. Refrigerate.
Fill a heavy frying pan with 1/4 inch canola oil and heat to 375 degrees F. Roll the chilled potato mixture into fish-stick shapes. Dip the fish sticks in the flour, then into the beaten egg, and then coat in the panko. Fry the croquettes in the hot oil until all sides are equally crispy and golden, 30 seconds.
For the spicy tartar sauce: Mix together the mayonnaise, onions, relish, capers, dill, vinegar, mustard, hot sauce and lemon juice in a large bowl and season with salt.
Serve the fish sticks with the tartar sauce.